Helium management control system

ABSTRACT

A helium management control system for controlling the helium refrigerant supply from a common manifold supplies a plurality of cryogenic refrigerators with an appropriate helium supply. The system employs a plurality of sensors to monitor and regulate the overall refrigerant supply to deliver an appropriate refrigerant supply to each of the cryogenic refrigerators depending on the computed aggregate cooling demand of all of the cryogenic refrigerators. An appropriate supply of helium is distributed to each cryopump by sensing excess and sparse helium refrigerant and redistributing refrigerant accordingly. If the total refrigeration supply exceeds the total refrigerant demand, or consumption, excess refrigerant is directed to cryogenic refrigerators which can utilize the excess helium to complete a current cooling function more quickly. Similarly, if the total refrigeration demand exceeds the total refrigeration supply, the refrigerant supply to some or all of the cryogenic refrigerators will be reduced accordingly so that detrimental or slowing effects are minimized based upon the current cooling function.

RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.11/590,673, filed Oct. 31, 2006 now U.S. Pat. No. 7,788,942, which is acontinuation-in-part of U.S. application Ser. No. 09/909,863, filed Jul.20, 2001 now U.S. Pat. No. 7,127,901. The entire teachings of the aboveapplications are incorporated herein by reference.

BACKGROUND

Vacuum process chambers are often employed in manufacturing to provide avacuum environment for tasks such as semiconductor wafer fabrication,electron microscopy, gas chromatography, and others. Such chambers aretypically achieved by attaching a vacuum pump to the vacuum processchamber in a sealed arrangement. The vacuum pump operates to removesubstantially all of the molecules from the vacuum process chamber,therefore creating a vacuum environment.

One type of vacuum pump is a cryopump, such as that disclosed in U.S.Pat. No. 5,862,671, issued Jan. 26, 1999, assigned to the assignee ofthe present application and incorporated by reference in its entirety.Cryopumps remove molecules from a vacuum process chamber by cooling asurface to temperatures approaching absolute zero. At such temperatures,most all gases condense on the cooled surface, called a cryogenic array,thereby removing substantially all molecules from the vacuum processchamber.

Cryopumps typically employ a helium driven refrigerator to achieve thenear absolute zero temperatures required. A compressor is used tocompress and pump the helium refrigerant to the cryogenic refrigeratorin the cryopump, and a cylindrical shaped vessel called a cold finger inthe cryogenic refrigerator receives the helium. A cryogenic array isattached to and in thermal communication with the cold finger and cooledtherewith. A displacer reciprocates inside the cold finger as the heliumexpands, driven by a displacer drive motor which reciprocates thedisplacer and regulates the quantity of helium used. As the heliumexpands in the cold finger, heat is drawn off the cryogenic array,generating the near absolute zero temperatures required to condensegases on the cryogenic array.

The amount of helium refrigerant available to the cryogenic refrigeratordetermines the rate at which cooling occurs. A greater supply of heliumdecreases the amount of time required for cooldown, which is the timerequired to achieve cryopumping temperatures. The rate of heliumconsumption also varies with the temperature of the cryogenicrefrigerator. As the cryogenic refrigerator becomes colder, a greatersupply of helium is required to continue the cooling process. In acryopumped vacuum process chamber, downtime can result in substantialeconomic impact, due to lost manufacturing time. Accordingly, thecapability to rapidly achieve and maintain cryopumping temperatures isbeneficial.

One prior art type of helium distribution is described in U.S. Pat. No.5,775,109, entitled “Enhanced Cooldown of Multiple CryogenicRefrigerators Supplied by a Common Compressor,” filed Jan. 2, 1997 andassigned to the assignee of the present application, incorporated hereinby reference in its entirety. This patent suggests individuallymonitoring the temperature of each of a plurality of cryopumps tocontrol the speed of each displacer drive motor when a cryopump attainsa triggering temperature. As cryopumps require varying amounts of heliumdepending upon the operation currently being performed, regulating thedrive motor speed can reduce or increase the helium supply accordingly.In this system, each cryopump monitors temperature and controls thedrive motor speed accordingly.

Frequently, however, a common helium supply manifold supplying aplurality of cryopumps is capable of supplying more helium than requiredby all of the cryopumps. Excess helium which is not identified is oftenunutilized, which can increase the time required for cooldown and whichcan cause a cryogenic refrigerator to become colder than needed, wastingpower and other resources required to maintain the helium refrigerantsupply.

SUMMARY

A method for controlling distribution of a resource, such asrefrigerant, among a plurality of consumers, such as refrigerators, isprovided by computing an available quantity of the refrigerant andcomputing a demand of the refrigerant by each of the plurality ofrefrigerators. The demand from the users is aggregated, and anallocation of the refrigerant based on the aggregated demand isdetermined for each of the refrigerators. Periodically, at regularintervals, the allocation of the refrigerant is redistributed byrecomputing the demand of each of the users by reevaluating a currentneed of each of the refrigerators.

In a system such as a cryogenic refrigeration system, the method ofcontrolling includes a compressor bank having at least one compressorand a plurality of cryogenic refrigerators supplied with refrigerantfrom the compressor bank. Management of the refrigerant supply from thecompressors to each of the cryogenic refrigerators is performed byidentifying the refrigeration requirements of each of the refrigerators,and, from a vacuum network controller, allocating a supply ofrefrigerant to the refrigerators according to the identifiedrequirements.

An embodiment of the helium management control system for controllingthe helium refrigerant supply from a common manifold supplies aplurality of cryogenic refrigerators with an appropriate helium supply.The system employs a plurality of sensors to monitor and regulate theoverall refrigerant supply to deliver a refrigerant supply to each ofthe cryogenic refrigerators depending on the aggregate cooling load ofall of the cryogenic refrigerators. Refrigerant demand for each of thecryogenic refrigerators is computed by the corresponding cryopump. Thetotal refrigeration capacity of the helium supply is apportioned to eachof the cryogenic refrigerators to optimize the refrigerant delivery. Anappropriate supply of helium is distributed to each cryopump by sensingexcess and sparse helium refrigerant and distributing the refrigerantaccordingly. If the total refrigeration supply exceeds the totalrefrigerant demand, excess refrigerant is directed to cryogenicrefrigerators which can utilize the excess helium. Similarly, if thetotal refrigeration demand exceeds the total refrigeration supply, therefrigerant supply to some or all of the cryogenic refrigerators will bereduced accordingly so that detrimental or slowing effects areminimized.

The refrigerant supply may be delivered from one or more compressors, orcommon compressor bank, to a plurality of cryogenic refrigerators via ahelium supply manifold. The refrigerant supply from each compressorcomprising the common compressor bank is used to determine therefrigerant supply. The total refrigerant demand, computed based on datafrom sensors attached to each of the cryopumps containing the cryogenicrefrigerators, is also computed depending on the particular operationthat the cryogenic refrigerator is performing. As certain operations mayconsume more refrigerant than others, a refrigerant supply is computedfor each of the cryogenic refrigerators. A cooldown function requiresthe most helium, and therefore will be afforded the maximum refrigerantsupply that can be delivered without disturbing other cryogenicrefrigerators. A regeneration function requires little or norefrigerant, and therefore will free up refrigerant for other cryogenicrefrigerators. During normal operation of one or more cryogenicrefrigerators, helium is delivered to attempt to keep the cryogenicrefrigerator in a state of equilibrium. Excess helium can be deliveredto cryogenic refrigerators in a cooldown state, or the total refrigerantsupply can be reduced if there is no demand for excess helium.

A variety of parameters are monitored by the system to compute theappropriate refrigerant supply for each cryogenic refrigerator. Suchparameters include computed refrigerant flow rate through the cryogenicrefrigerator, the speed of the drive motor, the pressure of therefrigerant, and the temperature of the cryogenic refrigerator. In thismanner, an appropriate refrigerant supply can be delivered to aplurality of cryogenic refrigerators from the common compressor bankdepending on the aggregate refrigerant load and the current coolingfunction of the individual cryogenic refrigerators. Therefore, thehelium management control system can minimize detrimental or slowingeffects from a sparse refrigerant supply and increase performance in thecase of an excess of refrigerant supply.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 a is a schematic illustration of a typical prior art cryogenicrefrigerator

FIG. 1 b shows a cutaway view of a typical prior art cryopump includingthe cryogenic refrigerator of FIG. 1;

FIG. 2 shows a block diagram of a cryogenic refrigeration system mastercontroller connected to a plurality of cryopumps and compressors;

FIG. 3 shows an example of helium refrigerant flow rate over time;

FIG. 4 shows a diagram of the helium consumption model used to determinethe quantity of helium a cryogenic refrigerator can consume;

FIG. 5 a shows a block diagram of the data and control flow of oneembodiment of the present invention;

FIG. 5 b shows a block diagram of the data and control flow of anotherembodiment of the present invention;

FIG. 6 shows a top level flowchart of the system master controller;

FIG. 7 shows a block diagram of the data flow between the mastercontroller, the compressors, and the cryopumps;

FIG. 8 shows a state diagram of the master controller;

FIG. 9 shows a flowchart of operation in the distribution per demandstate;

FIG. 10 shows a flowchart of the underpressure check routine;

FIG. 11 shows a flowchart of the distribution routing;

FIG. 12 shows a flowchart of the slave controller at the cryopump;

FIG. 13 shows a flowchart of the cooldown routine;

FIG. 14 shows a flowchart of the compressor check routine;

FIG. 15 shows a flowchart of helium management control in anotherparticular embodiment employing three states of control;

FIGS. 16 a-16 c show a flowchart of helium management control in aparticular embodiment employing four states, or modes, of control; and

FIGS. 17 a-17 b show a flowchart of helium management control in acryopump connected to the controller in FIGS. 16 a-16 c.

DETAILED DESCRIPTION

A description of example embodiments of the invention follows.

Prior to discussing helium management control, a discussion of cryopump(pump) operation may prove beneficial. Vacuum pumps such as cryopumpsand water pumps are used to drive a vacuum process chamber to near zeropressure. Near zero pressure, on the order of 10⁻⁶ to 10⁻⁹ ton or evenlower, is achieved by removing substantially all the molecules from thevacuum process chamber. The molecules are removed from the vacuumprocess chamber via the cryogenic refrigerator in the cryopump. Aportion of the cryogenic refrigerator is cooled to near absolute zero,typically between 10K-20K, causing substantially all of the molecules inthe process chamber to condense on the cryogenic array which is cooledby the cryogenic refrigerator. The cryogenic array is typically a set oflouvers and baffles which provide a surface area in a compact volume.The condensed gases are therefore reduced to a solid with a low vaporpressure so that a near vacuum is created. Further, the cryogenic arraymay include an adsorbent substance, such as charcoal, to adsorbmolecules which do not condense, such as hydrogen, helium, and neon. Thecryogenic refrigerator is powered by a refrigerant working fluid such ashelium gas, capable of achieving the temperatures approaching absolutezero.

Cryopumps consume varying amounts of helium depending upon their currentoperation and temperature. A series of pumps are connected to a commoncompressor bank of one or more compressors to maximize the availablehelium supply. Helium consumption by the pumps is monitored andregulated by a controller. By monitoring various operating parameters ofeach of the pumps, an appropriate supply of helium is supplied to eachpump. Excess helium is redirected to benefit pumps which can utilize it.Sparse helium is rationed so as to maintain operation and minimizedetrimental effects.

In the refrigerator of a typical cryopump, the working fluid iscompressed; the heat of compression is removed by air-cooled heatexchangers; the fluid is further cooled in a regenerative heat exchangematrix; and the gas is then expanded to produce cooling below theambient temperature. A cryopump must operate effectively at less than20K to remove gas molecules from the vacuum process chamber. Achievingthis low temperature requires the use of highly efficient heatexchangers and a working fluid such as helium gas that remains gaseousat temperatures approaching absolute zero.

The flow of compressed gas refrigerant in the cryogenic refrigerator ofa pump is cyclic. In the most basic form of a cryogenic refrigerator, asource of compressed gas, i.e., a compressor, is connected to a firstend of a cylinder through an inlet valve. An exhaust valve in an exhaustline leads from the first end to the low-pressure inlet of thecompressor. With a displacer including a regenerator positioned at asecond, cold end of the cylinder, and with the exhaust valve closed andthe inlet valve open, the cylinder fills with compressed gas. With theinlet valve still open, the displacer moves to the first end to forcethe compressed gas through the regenerator to the second end, the gasbeing cooled as it passes through the regenerator. When the inlet valveis closed and the exhaust valve is opened, the gas expands into thelow-pressure discharge line and cools further. The resulting temperaturegradient across the cylinder wall at the second end causes heat to flowfrom the load into the gas within the cylinder. With the exhaust valveopened and the inlet valve closed, the displacer is then moved to thesecond end, displacing gas back through the regenerator which returnsheat to the cold gas, thus cooling the regenerator, and the cycle iscompleted. In a typical pump, the cylinder is called a cold finger andit has a first stage and a second stage.

To produce the low temperatures required for cryopump operations, theincoming gas must be cooled before expansion. The regenerator extractsheat from the incoming gas, stores it, and then releases it to theexhaust stream. A regenerator is a reversing-flow heat exchanger throughwhich the helium passes alternately in either direction. The regeneratorcomprises a material of high surface area, high specific heat, and lowthermal conductivity. Thus, the regenerator will accept heat from thehelium if the temperature of the helium is higher. If the temperature ofthe helium is lower, the regenerator will release heat to the helium.

FIG. 1 a shows a block diagram of the cryogenic refrigerator 10internals. In the device of FIG. 1 a, helium enters the cold finger ofthe refrigerator through a high pressure valve 46 and exits through alow pressure valve 48. A displacer drive motor 216 drives displacers 207and 209 in the first stage and second stage of the cryogenicrefrigerator, respectively. The first stage displacer 207 includes afirst regenerator 211, and the second stage displacer 209 includes asecond regenerator 213. Heat is extracted from first-stage thermal load203, such as a cryopump radiation shield and frontal array, andsecond-stage load 205, such as a 10K-20K cryopanel.

FIG. 1 b shows a cutaway view of a cryopump including a cryogenicrefrigerator. In FIG. 1 b, the pump housing is removed to expose adisplacer drive 40 and a crosshead assembly 42. The crosshead convertsthe rotary motion of the motor 40 to reciprocating motion to drive adisplacer within the two-stage cold finger 44. With each cycle, heliumgas introduced into the cold finger under pressure through line 46 isexpanded and thus cooled to maintain the cold finger at cryogenictemperatures. Helium then warmed by a heat exchange matrix in thedisplacer is exhausted through line 48.

A first-stage heat station 50 is mounted at the cold end of the firststage 52 of the refrigerator. Similarly, heat station 54 is mounted tothe cold end of the second stage 56. Suitable temperature sensorelements 58 and 60 are mounted to the rear of the heat stations 50 and54.

The primary pumping surface is a cryogenic array 62 mounted to the heatsink 54. This array comprises a plurality of disks as disclosed in U.S.Pat. No. 4,555,907, incorporated by reference in its entirety. Lowtemperature adsorbent is mounted to protected surfaces of the array 62to adsorb noncondensible gases.

A cup-shaped radiation shield 64 is mounted to the first stage heatstation 50. The second stage of the cold finger extends through anopening in that radiation shield. This radiation shield 64 surrounds theprimary cryopanel array to the rear and sides to minimize heating of theprimary cryopanel array by radiation. The temperature of the radiationshield may range from as low as 40K at the heat sink 50 to as high as130K adjacent to the opening 68 to an evacuated chamber.

A frontal cryopanel array 70 serves as both a radiation shield for theprimary cryopanel array and as a cryopumping surface for higher boilingtemperature gases such as water vapor. This panel comprises a circulararray of concentric louvers and chevrons 72 joined by a spoke-like plate74. The configuration of this cryopanel 70 need not be confined tocircular, concentric components; but it should be so arranged as to actas a radiant heat shield and a higher temperature cryopumping panelwhile providing a path for lower boiling temperature gases to theprimary cryopanel.

FIG. 2 shows a bank of compressors used to supply helium refrigerant toa series of pumps. Referring to FIG. 2, the common compressor bank 16includes compressors 16 a-16 n which supply helium refrigerant to amanifold 18. The manifold 18 is connected to a series of pumps 10 a-10 nin conjunction with the slave controllers 215 a-215 n. The slavecontrollers each control a displacer drive motor 216 which drives adisplacer which reciprocates in the cold finger as the helium gasexpands. The displacer drive motor is used to regulate the cooling rateof the pump by the quantity of helium supplied. The vacuum networkmaster controller 12 (controller), or VNC, is connected to each of theslave controllers controlling the displacer driver motors 216 and isused to increase or decrease the quantity of helium refrigerant suppliedto the pump 10. Each of the pumps 10 has one or more sensors 14 a-14 nwhich provide feedback to the controller 12. The controller 12 thereforeregulates all the pumps 10 connected to it by receiving signals from thesensors 14 and computing a helium quantity for each pump 10 based on thesignals sent from the sensors 14 and from the total helium availablefrom the manifold, as will be described in more detail below.

It should be noted that the helium management control system isdescribed in conjunction with an exemplary cryogenic refrigerator in acryopump. The helium management control system may be used inconjunction with a helium supply driving a variety of cryogenicrefrigerators. A cryopump as described herein may, for example, be awaterpump, cooled by a single stage cryogenic refrigerator, such as thatdisclosed in U.S. Pat. No. 5,887,438, entitled “Low Profile In LineCryogenic Water Pump,” incorporated by reference in its entirety, andassigned to the assignee of the present application, or other heliumdriven cryogenic device.

Depending on the cooling operation of the pump, varying heliumconsumption rates occur. A cooldown operation brings the temperature ofthe pump from an ambient state down to the cryogenic temperatures, andrequires the most helium. Once cryogenic temperatures have beenachieved, a normal operating mode maintains the temperature and requiresa generally stable flow of helium. A regeneration operation warms up thepump to release accumulated, condensed gas and requires little or nohelium. Other factors can affect the helium consumption rate. Duringcooldown, the pump gradually consumes more helium as it becomes colder,approaching normal operating temperatures. At normal operatingtemperatures, vacuum process activities occurring in an attached vacuumprocess chamber may generate heat, increasing the refrigeration load,and in turn increasing the helium consumption rate.

The aggregate helium delivery rate of all the pumps connected to thecommon refrigerant supply can be used to determine an aggregate coolingdemand. Similarly, the refrigerant capacity of the compressor orcompressors contributing to the common refrigerant supply can be used todetermine a refrigerant capacity of the system. As indicated above, theactual consumption rate of each pump varies depending on a variety offactors. At a particular point in time, the refrigerant capacity of thesystem may exceed the aggregate refrigeration load, indicating excesshelium in the system. Similarly, if many pumps are experiencing a periodof high helium consumption, the aggregate refrigeration load may exceedthe refrigerant capacity, indicating helium sparsity.

By monitoring the current operation of all the pumps and the totalrefrigerant capacity, excess helium can be identified and diverted topumps which can utilize it. Similarly, sparse helium can be apportionedappropriately to maintain normal operation, or mitigate harmful effectsin extreme situations. For example, a cooldown operation can consume themost helium, and therefore the time required for cooldown can be reducedby diverting excess helium to pumps in cooldown. A pump in aregeneration operation requires little or no helium, and therefore canresult in excess helium being present. Also, a pump in normal operationmay begin to rise in temperature. In order to maintain cryopumpingtemperatures, helium may be diverted from a pump in cooldown, increasingcooldown time, but preserving cryopumping temperatures in the pump whichhad begun to warm up, to allow it to continue normal operations.

FIG. 3 shows an example of helium distribution flow rate over time. Eachof four pumps, 301-304 is shown over time shown by a horizontal axis. Atan initial time, all pumps are consuming equally. At the time shown bydotted line 310, pump 303 enters a regeneration state and warms up.Accordingly, additional helium can be provided to pumps 301, 302, and304. Alternatively, the drive motor speed of pumps 301, 302, and 304could be reduced to lower the overall helium draw from the commoncompressor bank, if increased helium would be inefficient. At the timeshown by the dotted line 312, the pump 303 has completed regenerationwarmup and enters a cooldown state. Excess helium is thereforeredirected from pumps 301, 302, and 304 to accelerate the cooldown ofpump 303. At the time shown by the dotted line 314, cryopump 303 hascompleted cooldown and all pumps return to an equal consumption rate atthe time shown by dotted line 316.

Helium consumed by the cryopumps is typically expressed in terms ofunits of mass flow rate, such as standard cubic feet per minute (SCFM),at a particular temperature and pressure. Other unit may also be used todenote the mass flow rate, such as grams/second. The helium consumed isdetermined from the maximum and minimum helium mass which is present inthe cold finger as the displacer reciprocates in a cyclic manner. FIG. 4illustrates the displacer positions for minimum and maximum heliummasses within the cold finger 44 in computing helium consumption ratesfor a cryogenic refrigerator in a cryopump. A displacer having a firststage and a second stage 207 and 209, respectively, reciprocates throughthe interior of the cold finger 44. As the displacer is reciprocated bythe drive motor 215, the helium is caused to expand, cooling the coldfinger. Each displacer cycle also opens the high pressure 46 (supply)and low pressure (exhaust) 48 lines to draw in unexpanded helium andexhaust expanded helium. The amount of helium which is consumed is givenby the formula:Flow Rate=(Maximum Mass−Minimum Mass)*Speed of Drive MotorTherefore, as the speed of the drive motor increases, the heliumconsumed increases because of increased displacer cycles, therebydrawing additional heat from the load.

For example, if a common compressor bank can deliver 84 SCFM of helium,the compressor bank could supply six refrigerators with 14 SCFM ofhelium: 84/6=14. As indicated above, the helium consumed by a pump canvary. If four of the refrigerators are only consuming 12.5 SCFM ofhelium, then there is 12.5*4, or 50 SCFM of refrigerant load from thosefour refrigerators. Since the compressor can supply 84 SCFM, there is84-50, or 34 SCFM for the remaining two refrigerators. If the remainingtwo refrigerators are in a cooldown state, they can each be suppliedwith 34/2, or 17 SCFM of helium due to the excess in the system. Inalternate embodiments, refrigerators in cooldown need not be apportionedan equal share of the excess helium.

FIG. 5A shows a block diagram of the data flow of an embodiment of thehelium management control system. Referring to FIG. 5A, each of thecompressors in the common compressor bank 16 sends an indication of themaximum helium available from each compressor to the controller 12,allowing computation of an aggregate helium supply. Each of the pumps 10sends the following parameters to the controller: a minimum heliumquantity, a current computed helium consumption rate, an operating mode,and a helium consumption status indicative of helium starvation. Thecontroller 12 sends an allocated helium parameter, or value, to thepumps indicative of the maximum helium rate at which the pump canconsume. The maximum helium consumption signal is used to regulate thedisplacer drive motor via the slave controller 215 connected to theparticular cryogenic refrigerator. As indicated above, the speed of thedisplacer drive motor regulates the helium consumption of the pump.

FIG. 5B shows a block diagram of the dataflow of another embodiment ofthe helium management control system. Similar to the system of FIG. 5A,each of the compressors in the common compressor bank 16 sends anindication of the maximum helium, or other refrigerant, available fromeach compressor to the controller 12, allowing computation of anaggregate helium supply. Each of the pumps 10 sends the followingparameters to the controller: a minimum helium quantity, a currentcomputed helium consumption rate, an operating mode, and a heliumconsumption status indicative of helium starvation. The controller 12 isconfigured to send an allocated helium parameter, or value, to the pumpsindicative of the maximum helium rate at which the pump can consume. Inthe system shown in FIG. 5B, the controller may also send an indicationto the compressor banks which may comprise variable speed compressors.Based on the indication, the compressors 16 may vary the supply ofrefrigerant to the cryopumps 10, whereby controlling the consumption ofthe refrigerant by the refrigerators. For example, the speed of thecompressors 16 may regulate the helium supply available to the cryopumps10 based on the parameters sent from the pumps 10 to the controller 12.

One of ordinary skill in the art will understand that refrigerantconsumption of each pump 12 in FIGS. 5A and 5B may be varied by a numberof methods, including adjusting the speed of the displacer drive motor,adjusting the length of the displacer stroke, adjusting the location ofthe displacer stroke, adjusting the displacer motion profile (displacerposition as a function of time) or altering the inlet valve timing tothrottle the refrigerant supply. Further, with respect to FIG. 5B, oneof ordinary skill will understand that the allocation of the refrigerantsupply from the compressors may be varied by different methods,including altering the number of compressors providing refrigerant tothe cryopumps, adjusting the compressor speed, altering the swept volumeof the compressors, or throttling distribution valves.

FIG. 6 shows a top level flowchart of control flow in the heliummanagement control system. The system polls, at a regular interval, todetermine if the helium supply to any of the cryopumps needs to beregulated. Alternatively, the system could be interrupt, or event,driven. When the polling interval expires, as shown at step 100, a checkis made to determine if all cryopumps are operating normally, asdepicted at step 102. If all pumps are operating normally, the systemawaits the next polling interval, as depicted at step 104. If any of thepumps or the system is not operating normally, that is, if one or moreof the pumps has reached a limit of allowed consumption, or if thesystem differential pressure (DP) has decreased below a critical value,then helium management control is performed, described further below, asshown at step 106. Two embodiments of helium management control aredescribed below.

FIG. 7 shows a block diagram of data flow between the vacuum networkcontroller (VNC 12), or master controller, the compressor(s) 16, and theslave controllers 215 a-215 d at the pumps 10 a-10 d. The compressorsends the supply pressure, and the return pressure to the VNC 12. Thecompressor also sends an initial value of the helium which it can supplyin standard cubic feet per minute (SCFM). An underpressure check routine110 in the VNC, described further below with respect to FIG. 10,computes a helium available corrected 112 value. The helium availablecorrected value is periodically recomputed to approximate how muchhelium is available for allocation based on current consumption. Thisvalue may vary slightly around the rated supply value based oncompressor displacement and speed because of factors such as wear andtear and efficiency of the pumps 10 and compressors 16. The heliumavailable corrected 112 value is used in a distribution routine 114,described further below with respect to FIG. 11.

The computed, allocated helium value is sent to the slave controller 215a-215 d controlling each pump 10 a-10 d, as shown by arrow 116. Theslave controller determines a maximum displacer motor speed at which thedisplacer motor may run without exceeding the allocated helium value. Apump speed control loop in the slave controller also controls displacermotor speed as a function of the cryopump temperature, and may run themotor at a lower speed, but may not exceed the speed corresponding tothe allocated helium value. The pump speed control loop also allows thepump to freely consume helium up to a default allocation value,according to the temperature, in a standalone mode if it is not drivenby the VNC 12. The slave controller 215 then computes a heliumconsumption value indicative of the actual helium consumption, describedfurther below. As with the total helium available value, the heliumconsumption value may differ from the rated displacement value for thepump depending on factors such as the current operating conditions andwear and tear. The helium consumption value is sent back to the VNC 12for use in successive helium allocation computations, as shown by arrow118.

FIG. 8 shows a state diagram of the VNC 12. Referring to FIGS. 6, 7 and8, a state transition may occur at each polling interval 100 dependingon the operation of the system. An idle state 120 occurs during systemstartup and mapping, and performs initial readings and default valuesprior to starting the pumps 10 and compressors 16. An initial size checkis also performed to ensure that the compressors are adequately sizedfor the pumps 10 connected to the manifold. When at least one pump 10and one compressor 16 are started, the VNC 12 transitions to a monitorstate 122.

In the monitor state 122, the pumps 10 are polled by the VNC at eachpolling interval 100 to determine if any pumps 10 are operating at alimit status, described further below. A pump 10 operating at limitstatus is consuming at or near its maximum allowed consumption, and mayneed more helium to avoid warming up. A transition to the distributionper demand state 124 occurs when at least one pump 10 is reporting alimit status or when DP has dropped below a critical value. Distributionper demand 124 attempts to reallocate excess helium in the system inorder to provide more helium to pumps 10 at limit, described furtherbelow with respect to FIG. 9. If distribution per demand 124 cannotreallocate sufficient helium to bring the pumps 10 out of limit statussuch that DP is still low, the system will transition to either anoverload state 126 or a distribution per hierarchy state 128.

In the overload state 126, the VNC 12 will maintain the currentallocation to each pump because it has already reallocated as muchhelium as possible to overconsuming pumps. For example, if five of sixpumps are operating adequately, but a sixth is overconsuming due to afaulty bypass valve, reallocating more helium to the defective pump willonly deprive the other five operational pumps. The distribution perhierarchy state 128, on the contrary, pursues a more aggressiveapproach, and selectively shuts down pumps 10 according to a userspecified hierarchy. For example, if a pump is in cooldown, it may bebeneficial to terminate the cooldown operation to avoid compromisinganother pump which is currently active with a wafer payload, to tradedowntime with saving the payload. Since, however, the distribution perhierarchy allows the VNC to actually terminate operations, a user maynot want this feature enabled.

FIG. 9 shows a flowchart of the computations performed in thedistribution per demand state 124. Referring to FIG. 9, the distributionper demand state is entered, as shown at step 130. Each of the pumps 10and compressors 16 is polled to determine the current operatingparameters, as shown at step 132. The supply pressure and returnpressure are received, and is the same for all compressors connected tothe common manifold. The pump operating parameters include the currentcomputed helium consumption value, the current helium allocation value,the consumption state (status), either OK or limit, the current coolingmode, either “ON” (refrigerator running, temperature control function onor running in manual override of temperature control), “COOLDOWN”(refrigerator running to achieve a setpoint temperature), or “OFF”(refrigerator not running and not consuming helium), and the minimumhelium which the pump needs to operate. The cooling mode indicates thecurrent cooling operation being performed by the pump, and is set tocooldown during a cooldown, temperature control when the pump is beingcontrolled by the VNC, and “none” when the pump does not require anyhelium, such as during a regeneration operation.

A check is performed to determine if a stabilization time has expiredsince the last redistribution, as depicted at step 134. Thestabilization timer indicates how much time will be given to determineif a previous reallocation was effective, typically one minute. If thestabilization timer has not expired, control reverts to step 130 to waitfor the next polling interval. If the stabilization timer has expired,or if no stabilization timer has been set, then a check is performed todetermine if either the overload or distribution per hierarchy (DPH)states should be entered, as depicted at step 136. Overload or DPH willbe entered if an underpressure condition exists, the system is stilloverconsuming, and all pumps in cooldown are operating at their minimumhelium allocation value. An underpressure condition exists if DPobtained in step 132 is below a particular setpoint threshold, typically190 lbs/in². As described above, a typical operating DP is about 200lbs/in², corresponding to a supply and return pressure of 400 and 200lbs/in², respectively.

The system is overconsuming when the sum of the computed heliumconsumption from all pumps is greater than the current, or most recentlycomputed, helium available corrected 112 (FIG. 7) value. In a particularembodiment, the pumps are overconsuming when the sum of computed heliumconsumption exceeds the helium available corrected 112 by 5%.

The third condition is that all pumps reporting a cooldown mode arealready at their minimum helium allocation as reported in step 132. Thesystem will tend to drive down the allocated helium parameter for pumpsin cooldown to allow more helium for pumps in temperature control, untilthe minimum helium allocation is reached. When all pumps have reachedthe minimum helium allocation, there is no excess helium to apportion toother pumps.

If the pumps in cooldown are all at minimum helium, and theunderpressure and overconsuming checks are positive, then a check isperformed to determine if DPH is set up and enabled, as shown at step138. If DPH is set up and enabled, then the DPH state 128 is entered,otherwise overload state is 126 entered.

If the system does not yet need to transition to overload 126 or DPH128, the underpressure check routine 140 is entered to compute a newvalue for the helium available corrected, described further below withrespect to FIG. 10. The distribution routine, also described furtherbelow, is then entered, as disclosed at step 142. The distributionroutine recomputes a new allocated helium value for each of the pumps10. A check is performed to determine if the system has redistributedhelium sufficiently to allow the state to transition to monitor 122, asshown at step 144. If the stabilization timer has elapsed and no pumpsare reporting a limit condition, than the VNC transitions to the monitorstate 122 since none of the pumps are deprived of sufficient helium.Next, a check is performed to determine if all compressors or all pumpshave been shut off, as depicted at step 146. If there are either nocompressors or no pumps turned on, the system transitions to the idlestate 120. Finally, the newly computed values for helium allocation aresent to the pumps 10, as disclosed at step 148.

FIG. 10 shows the underpressure check routine of step 140 in moredetail. Referring to FIG. 10, the underpressure check routine 140 isentered, as shown at step 150. A check is performed to determine if thesystem is operating with an underpressure condition, as disclosed atstep 152. The check may include reading a flag set during step 136above, or it may recompute DP and compare it to the DP setpoint. If thesystem is still operating at underpressure, a flow correction factor isdecremented by a predetermined value, such as 0.01, as disclosed at step154. The flow correction factor is then multiplied by the current heliumavailable corrected value 112 (FIG. 7) to yield a new helium availablecorrected value 112, as shown at step 158, and control reverts to thedistribution per demand flowchart, as shown at step 162. In this manner,the computed helium available is reduced to allow the distributionroutine, described further below, to compute the helium allocation froma smaller supply. Successive iterations, therefore, will have the effectof driving down the computed available helium until the systemstabilizes or until a transition is made to overload 126 or DPH 128.

If an underpressure condition does not exist, then a check is performedto determine if the aggregate computed helium consumption for all pumpsis greater than or within a certain threshold of the helium availablecorrected value 112, as shown at step 156. If the aggregate computedhelium consumption is within a certain threshold, then the helium flowis sufficient and the flow correction factor is incremented by apredetermined value, such as 0.01, as disclosed at step 160, therebyincreasing the computed helium available. The helium available correctedvalue is then recomputed, as shown at step 158, and control reverts tothe DPD routine, as depicted at step 162.

FIG. 11 shows the distribution routine of step 142 (FIG. 9) in moredetail. Referring to FIG. 11, the distribution routine is entered, asshown at step 164. The new allocated helium value for pumps in atemperature control mode is computed, as shown at step 166. The computedhelium consumption value reported by each pump is multiplied by thedelta helium factor of step 156 above to attempt to supply more heliumto the pumps in temperature control. In a particular embodiment, thedelta helium factor is 1.08. The helium available to apportion among thepumps in cooldown is then computed, as shown at step 168. The heliumconsumption value computed in step 166 is summed for all pumps intemperature control, and subtracted from the current value for heliumavailable corrected to yield the helium available for cooldown.Therefore, all pumps in temperature control will be considered first,and the remainder apportioned among the pumps in cooldown. The heliumavailable for cooldown is divided by the number of pumps in cooldown, asdepicted at step 170, and is weighted to accommodate the relative sizeof the pumps if there are different sizes currently attached to themanifold. Further, if the computed helium allocation is less than theminimum helium allocation for a particular pump, then the minimum heliumallocation will be used. Therefore, the system will attempt toredistribute additional helium to pumps in temperature control in orderto alleviate the limit state in the one or more pumps reporting such astate. Control then reverts to the distribution routine, as shown atstep 172.

FIG. 12 shows a top level diagram of the pump control flow. As indicatedabove, the pumps operate in one of three modes: temperature control,cooldown, and none, and two states: ok and limit. The pump also computesthe helium consumption value to report to the VNC. The pump slavecontroller periodically sends this information when requested by theVNC, and receives the allocated helium value from the VNC. The pumpspeed control loop flow then sets the maximum displacer speed (RPMs)accordingly. Note that the pump speed control loop operates in parallelto regulate displacer motor speed according to the first stagetemperature within the RPM range as computed by the pump control flow.In a particular embodiment, the pump speed control loop is a closed loopproportional-integral-differential (PID) loop.

Referring to FIG. 12, the pump control flow loop in the pump slavecontroller is entered, as shown at step 174. Entry is initiated by theVNC 12 (master), but could also be from asynchronous means such as aninterrupt driven mechanism. A check is performed to determine if thecurrent values of compressor supply and return pressure, used to computeDP, are valid, as depicted at step 176. Causes of invalid compressorvalues include communications failure between the pumps, compressor, andVNC, transducer failure, or the compressor being turned off. If thecompressor values indicate a possible problem, than the compressor checkroutine is entered, as disclosed at step 178 and described furtherbelow. If the compressor values appear valid, then a check is performedto determine if the pump was previously shut down by the compressorcheck routine, as depicted at step 180. If the pump was previously shutdown by the compressor check routine, then a power failure recovery isperformed, as depicted at step 182, to reinitialize, and the pumpcontrol loop is exited, as shown at step 194. If the pump was notpreviously shut down, than the current operating parameters are computedfor the pump, as disclosed at step 184.

The operating parameters are computed as follows: the helium consumptionparameter is computed to determine the current helium rate ofconsumption based on the first stage temperature, the second stagetemperature, the current displacer speed (RPM), supply pressure, returnpressure, and a pump constant based on the displacement of the pump(Cpumpconst)Helium Consumption=F(T1, T2, RPM, Psupply, Preturn, Cpumpconst)

A new allocation RPM value corresponding to the helium consumption valueis computed using the current allocated helium value sent from the VNC:Allocation RPM=(Allocated Helium*RPM)/Helium ConsumptionNote that the helium consumption value is also sent back to the VNC, asdescribed above, to compute a new value for the allocated helium value.The pump status of ok or limit, and the pump operating mode of cooldown,temperature control, and none are also computed, and sent to the VNC.

After computing the pump operating parameters, a check is performed todetermine if the pump is in a cooldown mode, as shown at step 186. Ifthe pump is in a cooldown mode, the cooldown routine is entered, asshown at step 188 and described further below. If the pump is not incooldown mode, then it is either on (in temperature control) or off, andthe maximum RPM is set to the lesser of allocation RPM, MaxRPM for thispump, or a constant global Maxrpm, typically 100 rpm, but not lower thanMinRPM, as described at step 190, and the pump control loop is exited,as shown at step 194.

FIG. 13 shows the pump cooldown routine. Referring to FIGS. 12 and 13,if the pump is in a cooldown mode, control passes to the cooldownroutine, as depicted at step 188. The cooldown routine is entered, asshown at step 400, and a check is performed to determine if the secondstage temperature is less than 17K. If the temperature is less than 17K,than a check is performed to determine if the first stage temperature iswithin 0.5K of a T1 setpoint, typically 100K. The T1 (first stage) isset to the normal expected operating temperature. If the T1 temperatureis sufficiently cold, than cooldown is complete, as disclosed at step406, and the cooldown routine is exited, as shown at step 422.

If the second stage temperature is not less than 17K, than a check isperformed to determine if the second stage temperature is less than 40K.If the second stage is less than 40, or if the first stage is not lessthan 0.5 K within the setpoint at step 404, then a check is performed todetermine if the allocation RPM is greater than 72 rpm, as shown at step408. If the allocation RPM is greater than 72, then it is set to 72 rpm,as shown at step 410. Therefore, the allocation RPM will be limited to72 when the second stage is less than 40K or when the second stage isless than 17K but the first stage has not yet dropped to the T1 setpoint+0.5K.

A check is performed to determine if the computed allocation RPM isgreater than MaxRPM, as shown at step 412. If it is, than current RPM isset to MaxRPM, as depicted at step 414, and the cooldown routine isexited, as shown at step 422. If allocation RPM is not greater thanMaxRPM, than a check is performed to determine if it is less thanMinRPM, as disclosed at step 416. If it is less than MinRPM, than RPM isset to MinRPM, as shown at step 420, otherwise it is set to allocationRPM, as described at step 418. The cooldown routine is then exited, asshown at step 422.

Returning to FIG. 12, step 178 shows the compressor check routine. FIG.14 shows the compressor check routine in more detail. Referring to FIGS.12 and 14, the compressor check routine is entered, as shown at step430. The main purpose of the compressor check routine is to determine ifthe compressors are functioning, and to shut the pumps down if they arenot operational. Default values for supply pressure, return pressure,and helium allocation are specified, as shown at step 432. Typicallythese values are defaults of supply pressure=400 psi, returnpressure=200 psi, and helium allocation=minimum helium, if the pump isin cooldown and the previous value for helium allocation if it is intemperature control.

A check is performed to determine if the compressor check routine wasentered during a previous iteration through the pump control flow loop,as disclosed at step 434. A compression check triggers a test timerwhich runs asynchronously from the pump control routine. Accordingly,multiple iterations through the compressor check routine will typicallyoccur as the pump is monitored over the test interval. If the compressorcheck routine was not previously running, a check is performed todetermine if the pump motor is on, as shown at step 436. If it is not,the compressor check routine is exited, as disclosed at step 454. If thepump motor is on, then a check is performed to determine if the pump isin a regeneration mode, as shown at step 438. If it is, then thecompressor check routine is exited, as disclosed at step 454.

If the pump is not in regeneration mode, then the current operatingmode, cooldown or on (in temperature control) is recorded, and a testtimer is set, as shown at step 440. The following iteration through thecompressor check routine will indicate that the compressor check routineis running, as depicted at step 434, and a check is performed todetermine if the test timer has expired, as shown at step 442. The testtimer is to allow a predetermined interval of time over which to monitorthe system for normal operation. If the test timer has not yet expired,then the compressor check routine is exited, as disclosed at step 454,to wait for the next iteration. If the test timer has expired, then acheck for cooldown mode is performed, as disclosed at step 444. If thepump is not in cooldown, than a check is performed to determine if thesecond stage temperature has risen above a predetermined threshold, asshown at step 448. In a particular embodiment, the threshold is 25K. Ifthe pump has not warmed past the predetermined threshold, then thecompressor is determined to be on and the pump is left operating, asdepicted at step 450. If the pump is not in cooldown, then a check isperformed to determine if the rate of cooldown is greater than apredetermined rate, such as 1K per minute, over the test timer interval,as described at step 446. If the rate of cooldown is not greater than1K/min, or if the second stage temperature has risen above 25K, then thecompressor is determined to be off and the pump is shut down, as shownat step 452. Control then passes to step 454 and the compressor checkroutine is exited until the next iteration.

Returning to FIG. 8, the distribution per hierarchy (DPH) state 128 canalso be used to deal with situations beyond normal operating conditions.This can be brought on by excessive heat load in the vacuum system or bythe degradation of a pump or compressor. The primary purpose ofdistribution per hierarchy is to allocate helium to the most importantpumps on the system while denying it to those of lesser importance. Insome cases, a pump should be shut down completely to allow helium to beused elsewhere. The system user must enable the distribution perhierarchy (DPH) function and define the relative importance of eachpump. For example, in a particular embodiment, a sputtering systemconsists of two load-lock chambers, a buffer chamber, a transferchamber, and four or more process chambers connected to the transferchamber. A process chamber pump might receive priority level 3, thetransfer chamber pump level 2, and the buffer chamber pump level 1. Aprocess chamber that has no wafer in it might be assigned priority level4. In case of a fault on the system that caused helium consumption toexceed demand, the tool controller would determine if any of the processchambers did not have a wafer in it, thus assigning them to level 3 or4. A process chamber without a wafer would have its allocated heliumreduced or the pump could be turned off to permit the other chambers tokeep operating. Part of the hierarchical system is to allow the systemto have a “soft crash.” That is, in a degrading system, wafers wouldhave time to finish processing and move out of process chambers, throughthe transfer chamber to the buffer chamber and back to the load locks asthe pumps were turned off behind them. The transfer time for such aprocess might be on the order of one to three minutes. The DPH state 128would first turn off the level 4 pumps one at a time, then level 3, andso on. If the fault condition goes away, the pumps could be turned onagain.

An assumption before entering the DPH state 128 is that there is nolonger any excess helium flow available on the system and therefore thesystem is beyond the distribution per demand state 124. Some pumps maybe operating acceptably with their allocation of helium, but at leastone pump has demanded more helium and none is available. Continuedoperation will result in the warmup of one or more pumps. Use of thepredetermined hierarchy imposed by the DPH state 128 will allow the VNCto perform “triage” to let the most important pumps keep cold whilesacrificing the other pumps. While three to five levels of priority maybe typical for a particular embodiment, the DPH state 128 should allowthe user to define the number of priority levels, including one levelfor each pump on the manifold.

The tool host controller may dynamically allocate the priorities basedon such issues as the presence or absence of a wafer. The user may alsodesire to maintain vacuum in a particular chamber until some conditionis fulfilled, like cooling down a very hot fixture. Users maypre-program whether a pump should be allowed to shut down entirely orallocated some minimum amount of helium. Where pumps have been given thesame priority level, then the VNC will arbitrarily select one pump atthat level to shut down or re-allocate helium. Action on other pumps atthe same or higher levels may be required until system stability isachieved.

The VNC may also enter a DPH state during pump cooldown. On some tools,it may be desirable to ensure that one or more pumps gets to operatingtemperature first. Priority can be given these pumps by using logic togive higher allocations to these pumps during cooldown relative to otherpumps on the map.

The VNC will accept hierarchy assignments from the tool host computerand store them. In the event of a problem while DPH is enabled, the VNCwill use the currently-assigned priorities to control the pumps.Alteration of priority levels by the tool host should be accepted by theVNC while DPH is in operation to deal with rapidly changing situations.

In another particular embodiment, the helium management control systememploys three modes of control, depicted in FIG. 15. This system uses apressure differential DP to determine the mode of control. Referring toFIGS. 1 b and 15, the pressure differential is the difference inpressure between the high pressure supply 46 line and the low pressureexhaust 48 line. In a typical cryopump, the high pressure supply 46 lineis at about 400 psi, and the low pressure exhaust 48 line is at about200 psi. The pressure differential is the difference between these twolines, and is typically about 200 psi. In extreme situations, if manycryopumps are consuming helium at a high rate, the pressure differentialcan drop below a critical threshold whereby the refrigeration capacitybegins to degrade sharply. It is one object of the present system toprevent the pressure differential from falling to the criticalthreshold.

In the system depicted in FIG. 15, the control modes are determined asfollows: a normal mode occurs when the pressure differential is at from190 to 205 psi; an under pressure mode occurs when the pressuredifferential is below 190 psi; an over pressure mode occurs when thepressure differential is above 205 psi. It should be noted that theseranges are approximate, and could be tuned in an actual system toprovide alternate ranges of the pressure differential corresponding tothe control modes.

Continuing to refer to FIG. 15, a flowchart shows the operation of thehelium management control system using three modes of control: normal,over pressure, and under pressure. When the pressure differential fallsoutside between 190 psi and 205 psi, or normal mode, the speeds of thedisplacer drive motors will be controlled to attempt to bring the systemback into a normal mode. Each of the cryopumps has a temperaturesetpoint. The setpoint of the cryopump is the temperature that thedisplacer drive motor will attempt to achieve during normal cryopumpingtemperatures. Reducing the setpoint will tend to have the effect ofincreasing the speed of the displacer drive motor to consume more heliumand reduce the temperature of the cryopump. Similarly, increasing thesetpoint will allow the cryogenic refrigerator to warm, tending toreduce the speed of the displacer drive motor and therefore consume lesshelium. The setpoint is used internally by the cryopump to vary thespeed of the displacer drive motor to match the temperature of the firststage of the cold finger to the setpoint, using closed loop control orother electronic control mechanism in the cryopump. Further, thesetpoint and the displacer drive motor speed both have an operatingrange beyond which the motor speed and setpoint may not be furthermodified.

More specifically, a polling interval expires, as depicted at step 300,and the system begins another check cycle. A check is made to determineif the system is currently in over pressure mode, as shown at step 302.If the system was not in over pressure mode, then a check is made todetermine if the system is in under pressure mode, as disclosed at step304. If the system was not in under pressure mode, then a check is madeto determine if the pressure differential is greater than 205 psi, asdepicted at step 306. If the pressure differential is not greater than205 psi, then a check is made to determine of the pressure differentialis less than 190 psi, as shown at step 308. If the pressure differentialwas not less than 190, then control reverts back to step 310 until thenext polling interval expires. The dotted line 312 outlines the sequenceof steps depicting normal mode operation, as just described. Thisiteration is repeated until the pressure differential falls outsidebetween 190 and 205 psi, described further below.

The system of FIG. 15 serves to lower temperature setpoints and increasemotor speed at 314, below, if the pressure differential is greater than205 psi. The temperature setpoints are increased and the motor speed isdecreased at 322, below, if the pressure differential drops below 190psi. After a change, the system is placed in an over pressure or anunder pressure mode for a time during which further changes are notpermitted in order to allow the system to stabilize.

At step 306, if the pressure differential is greater than 205, then apotential over pressure condition is occurring. An over pressurecondition is indicative of excess helium in the system. Dotted line 314generally depicts the over pressure corrective actions. In order toutilize the excess helium, the setpoint of all cryopumps not incooldown, and which are running, is decreased 2K, as disclosed at step316. The drive motor speed of any cryopumps in cooldown is increased by15 rpm, as shown at step 318. The system mode is set to over pressuremode to indicate that there is excess helium refrigerant capacity whichcan be utilized, as depicted at step 320. Note that there are minimumand maximum drive speed thresholds, described further below, which willkeep the drive motor speed within the predetermined operating range.

Continuing from above, at step 308, if the pressure differential is lessthan 190, than a potential under pressure condition is occurring. Anunderpressure condition is indicative of a sparsity of helium in thesystem. Dotted line 322 generally depicts the steps taken to correct anunder pressure condition. In order to conserve helium, the setpoint ofall cryopumps not in cooldown is increased 2K, as shown at step 324. Thedrive motor speed of the cryopumps in cooldown is decreased by 15 rpm,as disclosed at step 326. Decreasing the speed of the cryopumps in acooldown state will tend to lengthen the cooldown time, but will free upexcess helium to correct the under pressure condition and allow thepumps operating at normal cryopumping temperatures to continueoperation. The system mode is then set to under pressure, as depicted atstep 328, to indicate that an under pressure condition exists.

Continuing from above at step 304, if an under pressure conditionalready exists, a check is performed to determine if the under pressuremode has persisted for greater than one minute, as disclosed at step330. If the current under pressure mode has not persisted for more than1 minute, control reverts to step 310 to wait for the next pollinginterval to avoid system thrashing. If the current under pressure modehas persisted for more than one minute, then a check is performed todetermine if the current pressure differential DP is less than thepressure differential which caused under pressure mode to be entered, asdepicted at step 332. If under pressure mode has been previouslyentered, than the system should be starting to raise the pressuredifferential, otherwise there is a need for more aggressive heliummanagement. If the pressure differential DP is not less than the readingwhich caused under pressure mode to be entered, than a check isperformed to determine if the current under pressure mode has persistedfor ten minutes, as disclosed at step 334. If not, control reverts tostep 310 to wait for the next polling interval. The system thereforeallows ten minutes for the system to return to a normal pressuredifferential range before pursuing more aggressive helium management.

If the pressure differential is continuing to fall, or if ten minuteshave elapsed since under pressure mode was entered, the system exitsunder pressure mode, as disclosed at step 336. Under pressure mode isexited so that further corrective operations may occur at the nextpolling interval, described further below. Control reverts to step 310,and at the next polling interval, shown at step 300, the check at step304 will indicate that the system is not in under pressure mode.Accordingly, the pressure differential check at step 308 will indicatethat the pressure differential is still below 190, and the underpressure actions 324, 326, and 328 will recur, as described above.

Continuing from above at step 302, if an over pressure condition alreadyexists, then a check is performed to determine if the current overpressure mode has persisted for more than ten minutes, as shown at step338. If not, control reverts to step 308 for the low pressuredifferential check. If the current over pressure mode has persisted formore than ten minutes, then the system exits over pressure mode, asdepicted at step 340, and control reverts to step 300 to wait for thenext polling interval. The system falls out of over pressure mode totrigger the over pressure correction check. At the next pollinginterval, shown at step 300, since over pressure mode is no longer set,the over pressure mode check at step 302 will advance control to step306. If the pressure differential is still greater than 205, the overpressure actions of steps 316, 318, and 320 will recur, as describedabove.

In another particular embodiment, there are four control states ofhelium management at the controller, described further below, and threemodes. Briefly, the modes are setup, normal, and cooldown. Setup occursduring initial system setup to determine which compressors and cryopumpsare connected to the system. Cooldown mode indicates that one or morecryopumps is performing a cooldown operation. Normal mode occurs whenthe system has been started and all eryopumps have completed initialcooldown.

Each of the cryopumps attached to the system also has three heliummanagement operating modes which are reported to the controller. Atemperature control (TC) mode indicates that the cryopump is beingcontrolled by the controller. A cooldown (CD) mode indicates that thecryopump is performing a cooldown operation. A none (NONE) modeindicates that the pump is being allowed to freely consume helium as thedrive motor is permitted to run at a maximum speed.

The four control states of the helium management control system are eachgenerally indicative of a need for more aggressive helium management.The operating states are similar to the control modes described in theprevious embodiment. A normal state allows unregulated heliumconsumption by all cryopumps 10 in the system. A limit check stateoccurs when a pump is consuming equal to the maximum consumptioncomputed by the controller. A distribution per demand state occurs whena pump which reported a minimal supply continues to be starved after apredetermined threshold time. Distribution per demand causes excesshelium in the system to be redistributed, or for the maximum heliumsupply parameter for each cryopump to be reduced if there is no excess.If all pumps report helium starvation, a distribution per hierarchystate allocates helium to critical cryopumps per a predeterminedhierarchy by reducing helium to less critical pumps, which are allowedto warm.

The cryopumps also have a helium consumption status. An OK statusindicates that the cryopump is consuming less than 95% of the maximumhelium supply parameter. An APPROACHING status indicates that thecryopump is consuming greater than 95% of the maximum helium supplyparameter. A LIMIT status indicates that the cryopump is consuminghelium equal to the maximum helium consumption parameter. The heliumconsumption status is used to determine if a cryopump is consuming themaximum amount of helium needed to maintain cryopumping temperatures,and is therefore on the threshold of warming up. The approaching statusis not used to determine helium management control, but may be queriedby an operator as an informational item.

FIGS. 16 a-16 c show a flowchart of helium management control in thecontroller 12 as depicted in FIG. 2 in greater detail. Referring to FIG.16 a, an initial setup and mapping occurs, as depicted at step 610. Theinitial setup and mapping determines all the compressors 16 andcryopumps 10 connected to a common manifold 18. As indicated above instep 610, each cryopump sends a cryogenic refrigerator size, a minimumhelium supply, a helium consumption rate, and a cooldown completiontime, to be stored in the controller. The controller also receives thehelium supply available from each compressor 16. If there isinsufficient helium available to support at least the minimum heliumsupply for each pump, operation terminates. An initial heliumdistribution is computed, based on a proportional distribution accordingto cryopump size, and a maximum helium consumption signal is sent toeach cryopump. The controller may also read initial setup parametersindicative of a distribution hierarchy, described further below, andother operating parameters and defaults.

The controller then begins a control loop, receiving periodic input fromeach cryopump. Parameter signals, indicative of operating parameterdata, is received from each of the sensors 14, and a check is performedon the data received to determine if it is valid, as shown at step 612.Control reverts to step 612 until valid readings are obtained. A checkis performed to see if the distribution per demand state is active, asdisclosed at step 614. The distribution per demand state may be activeif a previous distribution per demand state was triggered, describedfurther below with respect to FIG. 16 c.

If the distribution per demand state is not active, then a check isperformed to determine if the limit check state is active, as shown atstep 616. The limit check state may be active if a previous limit checkwas positive. If the limit check state is active, control reverts tostep 620, described further below with respect to FIG. 16 b. If thelimit check state is not active, than the current consumption status foreach cryopump is examined, as depicted at step 618. For any pump whichis not in a cooldown state, the current consumption rate is examinedagainst the maximum helium consumption for that cryopump to determine ifa limit has been reached. Alternatively, the limit may be a percentageof the maximum helium consumption, such as 95%, in order to run thesystem in a more conservative manner. If the limit has been reached byone or more of the cryopumps, control reverts to step 620, describedfurther below with respect to FIG. 16 b.

If the limit has not been reached, then a check is performed todetermine if any of the cryopumps are in the cooldown state, as depictedat step 622. If none of the cryopumps are in the cooldown state, thesystem status is set to normal, as shown at step 626, and controlreverts to step 612 for the next control loop iteration.

If any of the cryopumps are in the cooldown state, the system mode isset to cooldown, as shown at step 624. A cooldown operation occurs aftera regeneration, or during initial system startup, and brings thecryogenic refrigerator back down to normal operating temperatures afterbeing warmed. A cooldown state consumes more helium than a normal state.Accordingly, the system is then examined for excess helium as in thehelium redistribution state. The helium margin for all pumps not in acooldown mode is computed and summed to determine an excess heliumvalue, as disclosed at step 686. A temporary maximum helium consumptionvalue is then computed for the cryopumps in cooldown, as disclosed atstep 688. If multiple cryopumps are in cooldown, the temporary maximumhelium consumption value is distributed proportionally according to thesize of the cryogenic refrigerator of each cryopump, as per the formuladescribed further below with respect to FIG. 17 b. Control then revertsto step 612 for the next control loop iteration.

Continuing from above, at step 620 the limit check state is entered.Referring to FIG. 16 b, a check is performed to determine if the limitcheck state is currently active, as shown at step 630. If it was notpreviously active, the time is marked as the initial time of the currentlimit check, as shown at step 632, and the system status is set to limitcheck, as depicted at step 634. If the limit check state is alreadyrunning, indicating that the system was already in a limit check state,then a timestamp is recorded as an ongoing limit check state, asdisclosed at step 636. Returning to FIG. 16 a, a check is performed todetermine if the current limit check state has persisted for more than apredetermined limit check threshold. In a particular embodiment, a checkis performed to determine if the current limit check threshold haspersisted for more than four minutes, as shown at step 638. If thesystem has not been in a limit check state for more than four minutes,then the limit check state is exited, as shown at step 638, and controlis passed to the cooldown check, as depicted at step 622. If the systemhas been in a limit check state for more than four minutes, then controlis passed to the helium redistribution routine, described further below,as disclosed at step 650. In this manner, the system is allowed a fourminute threshold for a limit check condition to correct before pursuingmore aggressive helium management.

Continuing from above, in FIG. 16 a steps 614 and 638, if a heliumredistribution is indicated, as shown at step 650, control istransferred to the helium redistribution routine, shown on FIG. 16 c.Referring to FIGS. 16 a and 16 c, a check is made to determine why thehelium redistribution state has been entered, as shown at step 652. Ifthe helium redistribution state was not already active, then a newhelium redistribution computation needs to occur because a previouslimit check state did not correct itself within four minutes. A check isperformed to determine if any pumps are reporting a helium consumptionstate of OK, as depicted at step 662. If at least one pump is reportingOK, and not LIMIT, helium redistribution is performed using the lessaggressive distribution per demand computation. In this context, one ofthe cryopumps is consuming helium equal to the maximum consumption valueand will warm up unless action is taken. The system state is set todistribution per demand, as depicted at step 664, and the controllerexamines the set of operating parameters for each cryopump. Theoperating parameters include current helium consumption, the maximumhelium consumption, the helium consumption status (OK, APPROACHING, orLIMIT), and cryopump operating mode TC (temperature control), CD(cooldown), or NONE), and cooldown completion time if operating mode wasCD.

An average helium margin is computed from the operating parameters foreach pump, indicative of the difference between the current consumptionof helium and the maximum consumption allowed for each pump, as depictedat step 666. The average helium margin, indicative of excess helium inthe system, is used to compute a new maximum consumption value for eachpump, as indicated at step 668, according to the following formula:

For each cryopump:Helium Margin=Max Consumption−Current Consumption

Calculate average margin:Average Margin=sum(Helium Margin)/# of Cryopumps

For each cryopump:HeMax=Current Consumption+Average Margin

Calculate new system total helium max consumption:Total System Max=sum(He Max)

For each cryopump:

${{New}\mspace{14mu}{Max}\mspace{14mu}{Consumption}} = {{{Max}\mspace{14mu}{Consumption}} + \frac{\left( {{{Total}\mspace{14mu}{System}\mspace{14mu}{He}\mspace{14mu}{Available}} - {{Total}\mspace{14mu}{System}\mspace{14mu}{Max}}} \right)}{\#\mspace{14mu}{of}\mspace{14mu}{Cryopumps}}}$Therefore, excess helium is distributed by setting a new maximumconsumption for each cryopump based on total helium available from thecommon manifold and the aggregate current maximum consumption for allthe cryopumps. A time stamp indicative of the time of reallocation iswritten, as shown at step 670. Control then reverts to the cooldowncheck in FIG. 16 a at step 622, as shown at step 658.

If the distribution per demand state was already active, then atimestamp is recorded as an on ongoing helium redistribution operation,as depicted at step 654. A check is performed to determine if more thana predetermined redistribution threshold has elapsed since the currentdistribution per demand state was entered. In the particular embodimentshown, the predetermined redistribution threshold is ten minutes. If thecurrent distribution per demand state has not been in effect for atleast ten minutes, control reverts back to the main control loop at thecooldown check 622 (FIG. 16 a), as shown at step 658. If the heliumredistribution mode has been in effect for at least ten minutes, thenthe redistribution is presumed to have effectively redistributed thehelium, and system state is set to normal, as depicted at step 660, sothat the main loop in FIG. 16 a can continue to monitor at regularintervals. In this manner, each iteration through the heliumredistribution routine provides ten minutes for the redistribution totake effect on the system. If the redistribution was not aggressiveenough, the helium redistribution state will again be entered andrecalculated to provide more aggressive helium management, until thesystem reaches a state of equilibrium.

If no pumps were reporting a helium consumption status of OK at step662, then all pumps had reached their maximum consumption limit, andhelium redistribution is performed using the more aggressivedistribution per hierarchy computations. In this context, no pumps arereporting a status of OK, and therefore all pumps are at a LIMIT status,indicative of no excess helium in the system. The operating parameters,enumerated above, are read from each cryopump and used to determine newmaximum helium consumption and possibly shut down one or more cryopumps.

The system state is set to distribution per hierarchy, as depicted atstep 672. The current operating mode of each pump is examined, as shownat step 674. A check is made to determine if any pumps were found whichwere not in temperature control or cooldown, as disclosed at step 676.If any pumps were found not in TC (temperature control) or CD(cooldown), they are placed in one of these states, as depicted at step678, and control reverts to step 612, FIG. 16 a to wait for the nextcontrol interval, as shown at step 680.

If all pumps are in either temperature control or cooldown, cryopumpsmust be selected to warm up or decrease their rate of cooldown. Acryopump hierarchy is read, as shown at step 682, to determine whichcryopumps are most critical and therefore will receive a sustainedsupply of helium. The cryopump hierarchy is a site-specific organizationof the priority of cryopumps which should be maintained at cryopumpingtemperatures. The hierarchy may be modified dynamically based on theactivities occurring in the vacuum process chambers connected to each ofthe cryopumps. Cryopumps concerned with critical processes, such as anexpensive semiconductor payload, for example, would typically continueto be supplied with helium. The cryopumps that are less critical asspecified in the hierarchy will be allowed to warm up or will decreasetheir rate of cooldown. Based on the hierarchy, a new maximum heliumconsumption value is computed for each cryopump, as shown at step 684.Control then reverts to step 622 for the cooldown check, as shown atstep 658.

FIGS. 17 a-17 b show a flowchart of the cryopump operations. Referringto FIG. 17 a, the cryopump control loop begins at step 500. Theinformation sent from the compressor is checked to assure validity, asshown at step 500. The information sent from the compressor is comparedto a range of normal values. If the information sent is outside therange of normal values, the compressor check state is entered, as shownat step 502, to perform compressor diagnostics. Since a compressor iscooled by the helium it supplies, extreme readings can be indicative ofa potentially damaging condition, such as a lack of helium. Thecompressor check routine will determine if the cryopumps need to be shutdown. If the cryopump has been previously shut down by the compressorcheck routine, control reverts to step 500 until the system indicatesthat the cryopump may resume operation, as depicted at step 504. If theinformation sent from the compressor was valid, the cryopump verifiesthat it was not instructed to shut down by the compressor check routine,as shown at step 506. The compressor check routine is used to preventdamaging operation to the compressor from running without helium, butalso to avoid shutting down a pump due to a more benign problem, such asa defective sensor. If the pump was previously shut down by thecompressor check routine, a power fail recovery routine will be invokedto restart the pump, as shown st step 508.

The cryopump calculates a helium margin by determining the differencebetween the current rate of consumption and the maximum heliumconsumption sent from the controller. The cryopump then determines thehelium consumption status based on the margin, and also determines thecurrent cryopump operating mode, as disclosed at step 510. The cryopumpthen checks to see if it has been placed into a cooldown state, as shownat step 512. If the cryopump is not in a cooldown state, a check isperformed to see if the pump operating status is LIMIT, as shown at step516. A LIMIT operating status occurs when the pump is consuming heliumequal to the maximum helium consumption parameter sent from thecontroller. If the pump operating status is LIMIT, a new maximum heliumconsumption parameter will be computed and sent from the controller, asdescribed above. The cryopump will compute and set the drive motor speedto correspond to the maximum helium consumption parameter, as depictedat step 518. Alternatively, each cryopump has a minimum and maximumoperating range, which will take precedence if the computed drive motorspeed falls outside the range.

If the pump was placed into a cooldown mode, as shown at step 512, thepump cooldown routine is invoked, as disclosed at step 514. FIG. 17 bshows a flowchart of a cooldown operation. Referring to FIG. 17 b, acheck is performed to determine if the second stage temperature is lessthan 17K, as depicted at step 520. If it is, than a check is performedto determine if the first stage temperature is within 0.5K of thesetpoint, as shown at step 522. If the first stage temperature is within0.5K of the setpoint, then cooldown is complete, as shown at step 524,and control reverts to step 500 until the next polling interval.

If the second stage temperature is greater than 17K, or if the firststage temperature is not within 0.5K of the setpoint, cooldown iscontinuing and the cryopump can benefit from excess helium. A temporaryhelium maximum is computed to allocate the excess helium, according tothe following formula:System Excess=sum(Margin of all Cryopumps)Temp Max=Current Max Consumption+System Excess*(CryopumpSize/sum(Cryopump Size)The total excess computed above therefore, is divided proportionallyamong the cryopumps based on their size, and added to the currentmaximum helium consumption parameter, as disclosed at step 526. Notethat while all cryopumps are apportioned a share of the excess,alternative embodiments may apportion the helium according to analternate formula, such as apportioning excess helium only to cryopumpsin cooldown. A temporary drive motor speed is computed to correspond tothe new temporary maximum helium consumption parameter, also shown atstep 526.

The newly computed drive motor speed is then compared to the minimum andmaximum drive motor speeds, similar to step 518 above. A check isperformed to determine if the second stage temperature is greater than40K, as disclosed at step 530. If the second stage is 40K or warmer, acheck is performed to determine if the new temporary drive motor speedis greater than the maximum rpm, typically 144 rpm, as shown at step530. If the temporary drive motor speed is greater than the maximum rpm,then the drive motor speed is set to the maximum rpm, as depicted atstep 532. If the temporary drive motor speed is not greater than themaximum rpm, as examined at step 536, than the drive motor speed is setto the temporary drive motor speed, as shown at step 538. If thetemporary drive motor speed is less than the minimum rpm, then the drivemotor speed is set to the minimum drive motor speed, as depicted st step540.

If the second stage temperature is less than 40K, then a check isperformed to determined if the temporary drive motor speed is greaterthan 72 rpm, as shown at step 534. If the temporary drive motor speed isnot greater than 72 rpm, than the drive motor speed is set to the lowerof the temporary drive motor speed or the minimum rpm, as depicted abovein step 536. If the temporary drive motor speed is greater than 72, thenthe drive motor speed is set to 72 rpm, as shown at step 542. In thismanner, the drive motor will tend to run at the temporary drive motorspeed or at the maximum speed until the second stage cools to 40K, andwill then tend to run at the temporary drive motor speed or at 72 untilcooldown is complete.

Those skilled in the art should readily appreciate that the programsdefining the operations and methods defined herein are deliverable to ahelium management control system in many forms, including but notlimited to a) information permanently stored on non-writeable storagemedia such as ROM devices, b) information alterably stored on writeablestorage media such as floppy disks, magnetic tapes, CDs, RAM devices,and other magnetic and optical media, or c) information conveyed to acomputer through communication media, for example using basebandsignaling or broadband signaling techniques, as in an electronic networksuch as the Internet or telephone modem lines. The operations andmethods may be implemented in a software executable object out of amemory by a processor or as a set of instructions embedded in a carrierwave. Alternatively, the operations and methods may be embodied in wholeor in part using hardware components, such as Application SpecificIntegrated Circuits (ASICs), state machines, controllers or otherhardware components or devices, or a combination of hardware andsoftware components.

While the system and method for controlling helium distribution havebeen particularly shown and described with references to embodimentsthereof, it will be understood by those skilled in the art that variouschanges in form and details may be made therein without departing fromthe scope of the invention encompassed by the appended claims.Accordingly, the present invention is not intended to be limited exceptby the following claims.

While this invention has been particularly shown and described withreferences to example embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

1. A system for controlling the supply of refrigerant to a plurality ofrefrigerators comprising: a compressor bank including at least onecompressor, the compressor including a high pressure supply line and alow pressure exhaust line, the compressor bank capable of supplying arefrigerant; one or more sensors attached to the high pressure supplyline and low pressure exhaust line to determine a differential pressurebetween the high pressure supply line and the low pressure exhaust line;a plurality of refrigerators coupled to the compressor bank, therefrigerators adapted to consume the refrigerant; and an electroniccontroller capable of determining a mode of control in accordance withthe differential pressure.
 2. The system of claim 1 wherein the mode ofcontrol comprises normal, over pressure and under pressure controlmodes.
 3. The system of claim 2 wherein the compressor bank is capableof reducing the total supply of refrigerant to the refrigerators inresponse to the over pressure control mode.
 4. The system of claim 2wherein the compressor bank is capable of increasing the total supply ofrefrigerant to the refrigerators in response to the under pressurecontrol mode.
 5. The system of claim 2 wherein the compressor bank iscapable of maintaining substantially constant total supply ofrefrigerant to the refrigerators in response to the normal pressurecontrol mode.
 6. The system of claim 1 wherein the refrigerators arecapable of altering refrigerant consumption in response to thedifferential pressure reaching a differential pressure set point.
 7. Amethod for controlling the supply of refrigerant to a plurality ofrefrigerators including: a compressor bank including at least onecompressor, the compressor including a high pressure supply line and alow pressure exhaust line, the compressor bank capable of supplying arefrigerant; one or more sensors attached to the high pressure supplyline and low pressure exhaust line to determine a pressure differentialbetween the high pressure supply line and the low pressure exhaust line;a plurality of refrigerators coupled to the compressor bank, therefrigerators adapted to consume the refrigerant; the refrigerantcomprises helium; an electronic controller coupled to the one or morepressure sensors, the method comprising: determining a pressuredifferential based on the output of the one or more sensors associatedwith the high pressure supply line and the low pressure exhaust line;determining an over pressure mode of control in accordance with thedifferential pressure, wherein the over pressure mode of control isindicative of excess supply of the refrigerant from the compressor bank;reducing the excess supply of the refrigerant from the compressor bank.8. The method of claim 7 wherein reducing the excess supply of therefrigerant from the compressor bank further comprises decreasing thespeed of the at least one compressor.
 9. The method of claim 7 whereindetermining an over pressure mode of control further comprises comparinga differential pressure set point with the determined differentialpressure.
 10. The method of claim 9 wherein the differential pressureset point is tuned to provide alternative differential setpoints for thecorresponding mode of control.
 11. A method for controlling the supplyof refrigerant to a plurality of refrigerators including: a compressorbank including at least one compressor, the compressor including a highpressure supply line and a low pressure exhaust line, the compressorbank capable of supplying a refrigerant; the refrigerant compriseshelium; one or more sensors attached to the high pressure supply lineand low pressure exhaust line to determine a pressure differentialbetween the high pressure supply line and the low pressure exhaust line;a plurality of refrigerators coupled to the compressor bank, therefrigerators adapted to consume the refrigerant; an electroniccontroller coupled to the one or more pressure sensors, the methodcomprising: determining a differential pressure in accordance withreading the output signals from the one or more sensors; determining amode of control in accordance with the differential pressure, whereinthe mode of control comprises normal, over pressure and under pressurecontrol modes; varying the supply of the refrigerant from the compressorbank in accordance with the determined mode of control; wherein thesupply of refrigerant from the compressor bank is increased as a resultof increasing the speed of the at least one compressor while thedetermined mode of control is under pressure and the supply ofrefrigerant from the compressor bank is decreased as a result ofreducing the speed of the at least one compressor while the determinedmode of control is over pressure.